It is well known that the carbon-carbon double bonds in polymers may be hydrogenated by treatment of the polymer with hydrogen in the presence of a number of catalysts. It is also well known that the carbon-carbon double bonds in a nitrile rubber, the nitrile rubber being a polymer comprising a C.sub.4 -C.sub.6 conjugated diolefin and a C.sub.3 -C.sub.5 unsaturated nitrile, can be selectively hydrogenated, without significant hydrogenation of the C.ident.N bonds, by treatment of the polymer with hydrogen in the presence of selected catalysts--for example, British Patent 1,558,491; U.S. Pat. Nos. 3,700,637; 4,384,081; 4,464,515 and 4,503,196. The use of ruthenium catalysts for the hydrogenation of nitrile rubbers is described in U.S. Pat. Nos. 4,631,315; 4,816,525 and 4,812,528.
In the hydrogenation of nitrile rubbers, it has been found that the molecular weight of the polymer, as indicated by the measured intrinsic viscosity or the Mooney viscosity, increases--this molecular weight increase is believed to be due to a low level of interaction occurring between two or more polymer molecules. The increase in molecular weight varies with the nature of the catalyst, the solvent used in the hydrogenation process and the reaction conditions used for the hydrogenation. The molecular weight increase is particularly noticeable when certain of the ruthenium catalysts are used and, in fact, under certain conditions the interaction between polymer molecules can be such that the hydrogenated polymer contains gelled (crosslinked) or insoluble polymer. Although a slight increase in molecular weight can be tolerated, if the molecular weight of the hydrogenated polymer is too high this causes it to be of low acceptability by the manufacturer of the products, such as hoses, gaskets, etc., because it is difficult to handle such high molecular weight polymers on conventional equipment.
Accordingly, the present invention is directed to an improved process wherein the molecular weight increase in the hydrogenation process is minimized and controlled.